The goal of this project is to determine whether regional cerebral glucose metabolism, assessed by Positron Emission Tomography (PET), is abnormal in two subgroups of low IQ people (mildly retarded with unknown etiology: IQ between 55 and 70, n=10; and low normal: IQ 70 to 85, n=10) compared to Down's Syndrome (n=10) and to normals (n=20). All groups will be age (18-30 years old) and sex matched. The specific aim is to test three different conceptualizations of brain function in mental retardation: 1) The brain inefficiency model is based on previous PET data showing inverse correlations between glucose metabolic rate and psychophysical task performance in studies from three PET centers. It predicts higher than normal glucose rates in the mildly retarded. 2) The regional deficit model predicts specific areas of brain damage with lower than normal glucose rates for the retarded and Down's groups, especially frontal lobe and basal ganglia. 3) The disorganization model predicts the pattern of correlations among brain areas will differ among the groups. PET will use FDG as the metabolic tracer and the degraded Continuous Performance Test (CPT) of vigilance will be the uptake task for all groups. Before scanning, the low IQ groups will be trained to performance criteria on the CPT which will be adjusted by finding the degree of stimulus degradation necessary for each subject to attain d' in the normal range. Pilot data in low IQ subjects (n=ll) show the feasibility of a 32 minute CPT. Each subject will undergo MRI scanning as well for exact anatomical localization of brain areas. Statistical analyses will include repeated measures ANOVAs, correlations and the Kullback test.